1. Field of the Invention
The present invention relates to a variable current source which linearly changes a direct current or an alternate current in accordance with a control voltage which is supplied to an electronic circuit or the like from outside.
2. Description of the Background Art
Recent years have seen an increased need for rationalized manufacturing of electronic equipment such as a TV and a VCR. To meet this demand, more and more of manual adjustment of a variable resistor and other parts conventionally performed by a man are now replaced by unattended adjustment under the control of a microcontroller. In a manufacturing process controlled by a microcontroller, a digital control output of the microcontroller is converted by a D/A convertor or the like into a direct current control voltage which will control various circuits. However, it is difficult to fabricate such a variable current source which linearly changes a current in accordance with a control voltage and supplies the linearly changed current to a circuit to be controlled.
FIG. 7 is a circuitry diagram of a conventional variable current source. In FIG. 7, indicated at 1 and 2 are NPN bipolar transistors with their emitters connected in common to form a differential amplifier. Noted at 3 and 4 are NPN bipolar transistors which have their emitters connected to bases of the transistors 1 and 2, respectively. Bases of the transistors 3 and 4 are both connected to a voltage source 5 and collectors of the transistors 3 and 4 are both connected to a power source 30. A collector of the transistor 2 is also connected to the power source 30.
A collector of the transistor 1 is indicated at 6. Emitters of transistors 21 and 22 are connected to each other via resistors 23 and 24 and collectors of the transistors 21 and 22 are connected to emitters of the transistors 3 and 4, respectively, so as to form a differential amplifier.
Indicated at 25 is a constant current source which is connected between a common contact point of the resistors 23 and 24 and a ground, and indicated at 26 is a current source which is connected between the emitters of the transistors 1 and 2 and the ground. A voltage source which is connected to a base of the transistor 21 is noted at 27. A variable voltage source connected to a base of the transistor 22 is shown at 28. A current supply block 100 formed by the elements 21 to 27 supplies currents to the transistors 1 to 4. A Gilbert amplifier 101 formed by the elements 1 to 5 is generally known as an amplifier which determines the amounts of a current to be supplied to the transistors 1 and 2.
Now, operation of the variable current source of FIG. 7 will be described. Because of a voltage supplied by the variable voltage source 28, a current from the variable current source 25 is divided in the differential amplifier which is formed by the transistors 21 and 22 and the associated emitter resistors 23 and 24 so that emitter currents are supplied to the transistors 3 and 4. Here, assuming that collector currents flowing through the transistors 21 and 22 are IA and IB, respectively, resistances of the resistors 23 and 24 are both RE, a current value of the current source 25 is Io, voltage values of the voltage sources 27 and 28 are VA and VB, respectively, and a voltage across the differential amplifier is Vcont, ##EQU1## where K is a Boltzmann's coefficient, T is an absolute temperature and q is an electrical charge.
Thus, the emitter currents into the transistors 3 and 4 are IA and IB, respectively. If base-emitter forward voltages are VBE3 and VBE4, respectively, EQU VBE3=(KT/q)ln(IA/Is) (3) EQU VBE4=(KT/q)ln(IB/Is) (4)
A voltage .alpha.V impressed upon the differential amplifier which is formed by the transistors 1 and 2 is: ##EQU2## Hence, if the collector currents in the transistors 1 and 2 are I1 and I2, respectively, a ratio I1/I2 is determined by: ##EQU3## That is, where a current generated in the current source 26 is IE, the output current I1 is determined by: ##EQU4##
Thus, due to the control voltage Vcont, a current is divided into the collector currents which flow through the transistors 1 and 2 at the same ratio as that determined by Eq. 1 regarding the currents IA and IB.
Here, for example, if (KT/q) 1n (IB/IA)&lt;&lt;RE .vertline.(IB-IA).vertline.in Eq. 1, Eq. 1 can be simplified as: EQU Vcont=RE(IB-IA) (8)
Hence, from Eqs. 2 and 8, EQU IB/IA=(loRE+Vcont)/(loRE-Vcont) (9)
Therefore, from Eqs. 5 and 8, EQU I1/I2=(loRE+Vcont)/(IoRE-Vcont) (10)
The current I1 available from an output 6 which is connected to the collector of the transistor 1 is expressed by Eq. 11 from Eqs. 2 and 6. EQU I1=IE/2(1+Vcont/IoRE) (11)
Eq. 11 shows that the collector current I1 is in proportion to the control voltage Vcont when (KT/q) 1n (IB/IA)&lt;&lt;.vertline.RE (IB-IA).vertline..
Having such a structure as above, the conventional variable current source creates the variable current I1 which is proportion to the control voltage Vcont when (KT/q) 1n (IB/IA)&lt;&lt;.vertline.RE (IB-IA).vertline.. However, in a graph plotting a control characteristic of the conventional variable current source, a control curve becomes less linear where this condition is not satisfied.
FIG. 8 shows an example of a control characteristic of the conventional variable current source. As can be seen in FIG. 8, a curve L1 expressing a change in the output current I1 created by the conventional variable current source in accordance with the control voltage Vcont exhibits an excellent linearity in a section A1 of the graph where the condition (KT/q) 1n (IB/IA)&lt;&lt;.vertline.RE (IB-IA).vertline. is satisfied. However, in the other sections of the graph, the curve L1 is deviated from an ideal control curve L0.
The deteriorated linearity of the control curve is a problem. For instance, in a case as that shown in FIG. 9 where the brightness of a television screen is controlled by changing direct current bias voltages on outputs R, G and B of a television receiver by means of a variable current source, if the control area differs between the outputs R, G and B, the color phase on the screen will be shifted with a change in the brightness.
As shown in FIG. 9, video signals R, G and B amplified by a video amplifier 71 are clamped by voltages VCR, VCG and VCB at an R-clamp circuit 72R, a G-clamp circuit 72G and a B-clamp circuit 72B, respectively, and then applied on an electron gun of a CRT 73. By increasing or decreasing the clamp voltages VCR, VCG and VCB, the brightness of the screen brightened by the CRT 73 is adjusted.
The outputs R, G and B of the CRT 73 have different luminous efficacies. To deal with this, during manufacturing, different brightness voltages are used for R, G and B so that supply currents IR, IG and IB from variable current sources 74R, 74G and 74B which are respectively connected to the clamp circuits 72R, 72G and 72B are different from each other. With the supply currents IR, IG and 1B having different values from each other, clamp voltages are developed at load resistors RR, RG and RB and supplied to the clamp circuits. Thus, the clamp voltages are adjusted appropriately, whereby resulting white light has a proper phase.
A problem occurs when a user changes the brightness of the screen by further changing the brightness voltages. To change the brightness voltages, it is necessary to change control voltages VcontR, VcontG and VcontB for the variable current sources 74R, 74G and 74B, respectively. Here, if the control voltages VcontR, VcontG and VcontB are changed in an area where the linearity of the control curve is poor, a balance between R, G and B will be deteriorated and resulting white light will have an improper phase.